1. Field of the Invention
The present invention relates to a magnetic storage device, more particularly a hard magnetic disk device, with a magnetic storage medium and a head.
2. Description of the Related Art
Magnetic spacing between a head and a medium is measured in a magnetic storage device, more particularly a hard magnetic disk device in order to check whether its operation is normal. In prior arts disclosed by patent references 1 through 3, a signal including a plurality of signal components each with a different frequency component is recorded on a medium, and by obtaining its reproduction signal, magnetic spacing between a head and a medium is measured. In this case, which frequencies are used can be theoretically determined arbitrarily. However, actually, in many cases, frequencies which are within a range close to a frequency used in the device and a frequency which is equal to three times as high as the said frequency are selected and the flying height of the head is estimated using a component amplitude ratio corresponding to these two frequency components in the reproduction signal.
It is known that in this case, as the flying height increases, the amplitude of the higher frequency component (hereinafter called “third order component”) decreases at a higher rate than the amplitude of the lower frequency component (hereinafter called “first order component”).
FIG. 1 roughly shows how to measure the flying height of a head.
A write instruction of a digital signal in which [111100] is repeated is inputted to a magnetic storage device composed of a head 10 and a disk 11. After being written into the disk 11, this signal is read using the head 10. The read signal is Fourier-transferred and its frequency component is extracted. The reproduction signal read in this way contains much first order and third order components of a specific frequency. The flying height of the head 10 can be estimated using the respective amplitude of the first order and third order components.
Patent reference 1: Japanese Patent Application Publication No. H7-1618
Patent reference 2: Japanese Patent No. 2711207
Patent reference 3: Japanese Patent Application Publication No. 2004-14092
If the actual flying height is greatly higher than the predetermined value, the third order component should be greatly small theoretically. However, technically the third order component cannot be measured, which-is equal to or less than a noise level. Therefore, according to the publicly known method, the flying height is wrongly estimated lower than the actual flying height due to the influence of noise. This error can be never detected by monitoring only the component amplitude ratio corresponding to the two frequency components.